So there they are, Philip Seymour Hoffman and Albert Finney, emoting like nobody's business after they've buried the woman who was, respectively, their mother and their wife, in Before the Devil Knows You're Dead. There's a silence, and then Hoffman speaks - whereupon Finney slaps him. It's clearly a crucial moment, this explosion of violence, but I've yet to talk to anyone who could hear what Hoffman actually said.

The same goes for whole chunks of the dialogue in Charlie Wilson's War (Seymour Hoffman's an offender here, too) and even for patches of the otherwise beautifully spoken There Will Be Blood. And it's not just because these are American films, the accents and dialects impenetrable to the British ear: the Coen brothers' No Country for Old Men is set in rural Texas and every syllable is crystal clear. The film critic Anne Billson offers an explanation: "The Coen brothers are famously proud of their dialogue so they make sure you can hear it."

Nor is it to do with rapid-fire dialogue, as in Charlie Wilson's War. There's no film dialogue more speedy than that between Rosalind Russell and Cary Grant in His Girl Friday (the director, Howard Hawks, is said to have kept a metronome going on set to ensure that the cut-and-thrust never flagged) - but their speech is bell-like in its clarity.

So what's going on? Is this the latest thing in sound recording? And why is it so much worse in American films? In his novel, Engleby, Sebastian Faulks has his sociopathic protagonist complain that: "American cop thrillers are bad because you can't hear what they say, and the story often turns on a side-of-the-mouth remark, indecipherable, that the stand-in was a fake or that the plant was working for the others."

Does Faulks feel the same way himself? Well, he shares my gripe about that Finney/Hoffman scene; none of the people he saw Before the Devil Knows You're Dead with had a clue what Hoffman said either. "As for war films," he says, "you can never hear a thing." But on the whole he is more tolerant: "It's to do with getting that gritty feeling of the New York police or the drug dealer - you don't want them to sound like Henry Higgins: 'I say, darling, try looking under the chassis. I think you'll find the drugs are hidden there. Did you get that?'" It's a matter, Faulks argues, of "realism - and inaudibility is a small price to pay. I mean, in Scorsese films it's very hard to catch what's being said, but you sure get the message."

Not always: as the critics noted at the time, Michael Mann's Miami Vice was almost totally opaque. First, the mix was muddier than Glastonbury; second, Colin Farrell was allowed to wallow in unintelligibility; and third, what dialogue was audible was couched in the incomprehensible argot of the Hispanic drug lords. That hunger for "authenticity" is always a problem: Andy Harries, producer of The Queen, says he doesn't even try to understand what's being said on DVDs of The Wire or The Shield. He goes straight to the subtitles. That way, he can acclimatise himself to the slang. But some DVDs are just hard on the ear: the writer Tim de Lisle is just one of the people I spoke to who had to take the subtitle route with the Bourne Trilogy.

That's DVDs. Films proper, in state-of-the-art cinemas, should not be a trial. Robert Fox, who produced The Hours and Notes on a Scandal, thinks it's "an old-fashioned thing: diction. And it's veering on arrogance. Maybe it's not the actor's fault; it's the director's responsibility to do something about it. The thing is, Brando mumbled, Pacino mumbled, De Niro mumbled - now lots of people have joined the club. Philip Seymour Hoffman? I think it's a slight affectation, maybe. But the trouble is, I think no director says, 'We can't hear you.'" (One music supervisor told me that sometimes the problem is that the director is too close to the material - he knows the script so well, he thinks it's audible.)

In the end, though, it's the actors who deliver the lines. Producer Hercules Bellville (Sexy Beast) also points to Brando: "Mumbling began with Brando," he says, "but you could discern what was being said." (It was Frank Sinatra who first dubbed Brando "Mumbles", after Brando got the part of Sky Masterson that Sinatra cherished in Guys and Dolls.)

Bellville also fingers Robert Altman's penchant for overlapping dialogue as a culprit for the present state of affairs - that, and naturalism. "I've got no problems with 1940s films; then only Peter Lorre had a strange accent. Now there are all sorts of odd accents and dialects, and the colloquial is often rapid and slurred. But technically, there's no reason for it. The whole process of mixing sound effects, dialogue and music takes weeks, tweaking this, fixing that. It can't be carelessness - but I do share your frustration."

Film editor Guy Bensley, who worked on Fade to Black and The Importance of Being Earnest, has sat through many sound mixes. "It's an intense business," he says. "You've got the composer here, the guy with the sound effects machine there, all competing. And some directors like to create a sense of energy from having a great wallop of swirling sound - but at the cost of knowing exactly what's going on." His prime example? "I remember watching [David Fincher's] Seven and not hearing a thing for the first hour. It all took place in pouring rain, and that was all you could hear."

Bensley has himself edited a film - the as-yet unreleased Butterfly On a Wheel - in which a lovers' reunion is pointed up with a downpour; the words themselves didn't really matter, it was the emotion that counted. (This is presumably what Paul Thomas Anderson is up to with Jonny Greenwood's intrusive score in There Will Be Blood.) But surely no actor or director would deliberately wish to have their words misunderstood? Bensley agrees: "Though it would be lovely to say that someone has stood up in the dubbing theatre and fought for the integrity of their unintelligibility, the answer's no, no, no."

There's no real excuse for being unintelligible. If the dialogue really can't be heard, the film can be revoiced - though it is expensive. And today's technology means that you can take out every micro-second of soundtrack and re-build it. So perhaps the blame lies, as one director tells me, with "teenage American directors asleep in the dubbing room and strung out on drugs". (Mind you, Before the Devil Knows You're Dead was directed by Sidney Lumet, who is in his 80s, while Mike Nichols, who is in his 70s, made Charlie Wilson's War.)

The surest way of avoiding all this, of course, is to take the French and Italian approach. They record the final version after filming, in the dubbing theatre. There's no silence on set as the camera rolls, and the performers see shooting as time, in effect, for a spirited read through. So you can always hear Catherine Deneuve. Or can you? My French is not that good.

Of course, there are some people you can do nothing about. As Bensley texted me after our conversation: "Does anyone understand Sylvester Stallone as Rocky? Or Rambo? Or ever?" And that really is humanity's loss.